The present note will be restricted to remarks on the nature of the organisms which enter so largely into the composition of certain tertiary deposits; since the opinion once entertained of the animal nature of many infusoria, now regarded as true vegetables, materially affects the geological conclusions respecting the persistence of certain species of organisms through long periods of time, during which the mollusca, zoophytes, &c. underwent repeated mutations both in the species and genera. Thus, for example, the _polierschiefer_, or polishing-slate of Bilin, and the berghmehl of Tuscany, are described by Ehrenberg as masses of the siliceous shells of animalcules of such extreme minuteness, that a cubic inch of the stone contains upwards of forty millions; the infusorial earth of Richmond, in Virginia, in like manner, is stated to be made up of the siliceous skeletons of animalcules of infinitesimal minuteness. But later investigations have (I conceive) satisfactorily established, that the greater part of these fossil organisms belongs to the vegetable and not to the animal kingdom.[124] The whole of the figures in Plate IV. of the "Medals of Creation," described as living Infusoria, on the authority of Ehrenberg, are undoubted vegetables, belonging to the great botanical groups called _Diatomaceæ_ (from the angular segments into which they separate by partial division), and _Desmidiaceæ_.[125] The entire family of _Bacillaria_ belongs to this group. These simplest forms of vegetable structures abound in every lake or stream of fresh and brackish water, in every pool, or bay, and throughout the ocean, from the equator to the poles; they secrete siliceous envelopes, which present an endless variety of form and structure, and after the death and decomposition of the perishable tissues of the plants, remain as perfectly transparent colourless shields of pure silica; such are the _Gaillonellæ_, _Euastra_, _Closteria_, _Naviculæ_, _Synhedræ_, _Podospheniæ_, _Xanthidia_, &c., which constitute so large a proportion of the infusorial earths described by Ehrenberg and other authors.[126] [Footnote 124: In my little work on Recent Infusoria, entitled "Thoughts on Animalcules, or a Glimpse of the Invisible World revealed by the Microscope," I have expressed my conviction of the vegetable nature of these organisms, as a reason for omitting figures and descriptions of any of the species in a work on living fresh-water animalcules.] [Footnote 125: The name Diatomaceæ is restricted by M. Brébisson to those species which have a siliceous envelope, or cuticle; and that of _Desmidiæ_ to those which are not siliceous, but reducible by heat to carbon.] [Footnote 126: The reader interested in this subject should consult the beautiful work of Mr. Hassall on the Desmidiaceæ, published by Messrs. Reeve & Benham.]